Method and device for stacking flat mail items

ABSTRACT

In a stacker, a mail item to be stacked is propelled against a stop wall at a stacking point by a stacking roll. A controlled driven underfloor belt and a stack support are displaced from the stack as the stack grows. The mail item to be stacked meets the preceding, already stacked item at an acute angle. Before it is stacked the mass of each item is determined. Stacking forces are measured with a measurement device in a preceding test phase the target values for the stacking pressures and the braking curve of the stacking roll up until the mail items have reached the stop are defined and stored based on statistical analyses in accordance with the mass of the mail items to be stacked. The speed of the respective mail item is reduced according to the braking curve assigned to the determined mass of the mail item.

The invention relates to a method and a device for stacking flat mail items.

In the invention the mail items are transported in turn by a transport means into stackers and are stacked vertically in these. In each stacker a mail item to be stacked is transported to a stop wall by means of a stacking roll. The mail item to be stacked meets the preceding already stacked item at an acute angle. A controlled driven underfloor belt and a stack support coupled in a releasable manner to this or a support which can be driven separately is moved away from the stack as the stack grows.

Each mail item to be stacked is to be guided so that it comes up against the stop wall and its front and lower edge is aligned in a line to the items already stacked.

With a movement away of the part stack in accordance with DE 102 23 349 A1 from the stacking point to make space for the mail item to be stacked, the displacement selected is smaller than the determined thickness of the mail item to be stacked. For this reduction of the displacement in relation to the measured thickness value the starting point used is an average compressibility of the part stack. However if the actual value deviates significantly from the assumed value as a result of non-compressible mail items, there is still a disproportionate rise in the stack pressure since the stacking force is only measured after the mail item hits the stop in the stacking run and a corrective intervention can then be undertaken. In some cases the mail item does not reach the stop wall of the stacker. The spring effect of the mail item stack cannot be taken into account.

If, to avoid this effect, as also in EP 0743269 B1, the part stack is moved away from the stacking point by the full amount determined of the thickness+safety margin (measurement tolerances) the support function of the stacking roll is briefly relinquished. The result, particularly with large-format mail items is that the stack tips, which dramatically influences the quality of the stack. If the mail item has reached the stop wall in the stacker, stacking force measurement in the area of the stacking roll is used to determine the stacking force and in an adjustment process by moving the underfloor belt the stacking force is adjusted in a range which is defined by threshold values for the permitted stacking force range. There is also the fact that after the imprecise positioning of the mail item in the stacker, a force is introduced onto the mail item by the constantly rotating stacking roll in the direction of conveyance which can lead to damage, twisting and warping of the stacked item.

The object of the invention is to create a method and a device for stacking flat mail items in a vertical position in a stacker with which mail items with widely differing dimensions and masses can be stacked in a protective manner aligned on their front and bottom edges.

In accordance with the invention the object is achieved by the features of claims 1 and 6.

In each stacker in which the mail items are stacked in a vertical position, a mail item to be stacked is propelled against a stop wall at a stacking point by a stacking roll that can be displaced away from the stack in opposition to a spring force and that is driven in a controlled manner in a starting-braking mode. A controlled driven underfloor belt and a stack support coupled in a releasable manner to this or a support which can be driven separately is displaced from the stack as the stack grows. The mail item to be stacked in each case meets the preceding, already stacked item at an acute angle. Before it is stacked the mass of each item to be stacked is determined. Stacking forces are measured with a measurement device arranged in the area of the stacking roll on the item transport plane or on the stacking roll itself when the stacking roll has engaged with the mail item to be stacked. In a preceding test phase the target values for the stacking pressures and the braking curve of the stacking roll up until the mail items have reached the stop are defined and stored based on statistical analyses in accordance with the mass of the mail items to be stacked. The speed of the respective mail item is reduced by the stacking roll according to the braking curve that has been assigned to the determined mass of the mail item.

As a result of the displaceable stacking roll the stacking force never exceeds a value which prevents correct stacking regardless of the characteristics of the stack already located in the stacker without the support function being lost.

The mail items are braked independently of their dimensions and masses to reduce the kinetic energy in a defined way such that an optimally aligned stack is produced. The braking of the stacking rolls means that after positioning of the relevant mail item the force introduced onto the mail item in the direction of conveyance is reduced, which avoids damage or twisting of the stacked mail items.

Advantageous embodiments of the invention are set down in the subclaims.

Thus it is advantageous, if the thickness of the mail item to be stacked is determined and for stacking a mail item the underfloor belt and/or the stacking support is moved away from the stacking point by the measured thickness of this mail item plus or minus a correction value for setting the stacking pressure assigned and stored for the mass of the mail item and thereafter, for an actual stacking pressure measured at the stacking roll which deviates from the target stacking pressure for this item is moved to maintain the target stacking pressure. The target stacking pressure depends on the mass and the thickness of the current mail item to be stacked and the spectrum and the characteristics (mass, thickness, compressibility) of the previously stacked mail items.

It is also advantageous for the mass of the mail items to be determined indirectly by the dimensions of the mail item being measured, for the volume to be calculated from these dimensions and multiplied by a defined mass factor per volume.

In the braking process of the stacking roll the braking curve can advantageously be designed so that the speed of the circumference of the stacking roll at the point in time at which the mail item has reached the stop wall is inverse to the mass of the mail item and that the stacking roll is subsequently stopped if the subsequent mail item is far enough away for there to be sufficient time to accelerate the stacking roll again to its rated speed before the mail item has reached it.

If the subsequent mail item is less than a specific distance from the stacker, e.g. if the time for acceleration from 0 to rated speed is not sufficient, it is advantageous to accelerate the speed of the circumference of the stacking roll immediately back to the rated speed of the transport means after the stacking of the relevant mail item.

The invention will be explained below with reference to the drawing in an exemplary embodiment.

The figures show

FIG. 1 a schematic overhead view of a device for stacking, showing a mail item to be stacked,

FIG. 2 a diagram of the speed curve of the stacking roll over time,

FIG. 3 a diagram of the curve of the stacking force plotted against time and against the position of the stacking roll.

As shown in FIG. 1 the stacking roll 1 is arranged so that it can move in the direction of the mail item stack 18 to be created. This is implemented using a motion link 2. A linear arrangement is also possible.

The stacking roll 1 is positioned in its rest position by means of a spring 3 with a defined spring characteristic in the direction of a stop 4. This stop 4 is adjustable which allows calibration of the sensor. Mail items 19 arriving, depending on their thickness, cause a greater or lesser displacement of the stacking roll 1. The size of the displacement creates an increase in the reaction force of the spring 3 and thereby of the stacking pressure. If the characteristic spring curve is designed accordingly there is a guarantee that in this case the stacking pressure only increases to such an extent that all mail items securely reach the stop wall 15.

A further stop 5 prevents an overdeflection of the stacking roll 1 especially when thicker mail items 19 are transported in.

To avoid unwanted oscillations a damping element 6 is fitted to the motion link 2.

As can be seen from FIG. 1, the mail items 19 are transported in turn clamped between stacking belts 11, 12 in a vertical position in the direction of stacking roll 1. So that the items can be stacked, the transport belts 12 aligned in the direction of part stack 18 are deflected before the stacking device over a deflection roll 13, whereas the other stacking belt 11 is only deflected directly before the stacking roll 1 via a deflection roll 10. The mail item 19 is directed at an acute angle as a result of its kinetic energy and through friction forces along the stacking belt 11 to the stacking roll 1 to the stacking support 14 located there, if no other mail item has yet been stacked, or against the last stacked mail item of the part stack 18 and is then between held between this item and the stacking roll 1.

The stacking roll 1 has a direct drive 16. The maximum circumferential speed of the stacking roll 1 corresponds during the stacking phase to the transport speed of the mail items 19 to be stacked.

The stacking pressure which is generated between the first mail item of the stack 18 and the stacking roll 1 must in this case be great enough for each mail item to be securely transported between the stacking roll 1 and the stacking support 14 or for the uppermost mail item of the stack 18 to reach the stop wall 15, at which the residual kinetic energy of the mail item 19 is dissipated. The stacking roll 1 is displaced to a greater or lesser extent by incoming mail items 19 depending on the thickness of the item and the stack compressibility, so that the stacking pressure does not become too high when a mail item is added to the stack. The slightly higher stack pressure produced here in accordance with the spring characteristic of the spring 3 is measured and by displacement of a stack support 14 and/or of an underfloor belt 17 and thereby of the part stack 18 is set (adjusted) again to the required value.

For measurement of the stacking pressure a suitable sensor for detecting the stacking pressure as a reaction force to the stacking roll 1 is provided. The sensor can be a movement recorder 7 in the area of the motion link 2 which determines the stacking pressure with the aid of the spring characteristic (FIG. 1) and feeds it to the control 9. Another option is a force sensor connected in series with the spring 3. Because of the widely different characteristics of the mail items (e.g. dimension, mass, surface properties) the rotation of the stacking roll 1 is also accelerated and braked in a defined manner as a function of these significant characteristics of the mail items and the position of the incoming mail item. This enables heavy mail items 19 to be braked so that they are not too squashed. Because the stacking roll 1 does not run continuously, mail items 19 with sensitive surfaces such as plastic for example are not damaged. They are thus not bent out of shape which can lead to the subsequent mail items 19 not being able to enter the stacker.

The speed of the stacking roll 1 is controlled in accordance with FIG. 2 as a function of the mass of the mail item. If the mail item 19 is transported in the direction of the stacking roll 1, the stacking roll 1 is thus accelerated in good time independently of the mass of the mail item, so that the transport speed at the circumference is reached before the mail item 19 comes into contact with the stacking roll 1. If the mail item 19 is securely engaged with the stacking roll 1, the roll is braked. The delay, i.e. the braking ramp, is selected to be greater for mail items 19 with greater mass than for thin, light mail items 19. One the mail item 19 has arrived at the stop wall 15 of the stacker, after a short delay, to prevent bounce effects, the stacking roll 1, to avoid the stressing of the topmost stacked mail item by the rotating stacking roll 1, is fully braked over a longer period, provided no further mail item 19 is fed into the stacker within the defined interval. If a further mail item 19 follows within a minimum interval, the stacking roll 1 is not completely braked but continues to run at the speed reached at this point until the following mail item 19 has been transported far enough for the stacking roll 1 to have to be accelerated again in order to be up to transport speed in good time for the arrival of the mail item 19 at the stacking roll 1. The stacking pressure is controlled by displacement of the stacking support 14 and/or of the underfloor belt 17. Once the mail item 19 to be stacked has reached a defined position, the advance of the underfloor belt 17/stacking support 14 is calculated and started. The advancing is ended before the mail item 19 has arrived at the stacking roll 1.

The advance is calculated from the thickness of the mail item 19 and the stacking pressure which should be set when the mail item 19 is engaged with the stacking roll 1 and this is thus deflected by this mail item 19. The spring characteristic curve of the support of the stacking roll 1 is used to calculate the distance by which the underfloor belt 17 must move the item stack 18 away from the stacking roll 1 in order to modify the stacking pressure accordingly.

If the stacking pressure is to be increased, the advance is made up of the item thickness minus the distance which must be covered in order to increase to the desired stacking pressure.

If the stacking pressure is to be reduced, the advance is made up of the thickness of the item plus the distance to be covered in order to reduce the desired stacking pressure.

FIGS. 2 and 3 show the relationships when the stacking pressure is increased. As can be seen from the stacking pressure curves, the starting point used is an average stacking pressure Ft1, which was determined experimentally beforehand at a point in time t1. If a mail item is registered before the stacking roll 1, by moving the underfloor belt 17/stacking support 14 backwards away from the stacking point, the stacking pressure Ft11 suitable for this mail item 19 based on its mass is set for secure stacking. The mail item 19 then deflects the stacking roll 1 and the stacking pressure increases briefly to the limited value Ft2. Subsequently the item stack 18 is moved backwards by the underfloor belt 17 far enough for the stacking pressure F_(t1) to be re-established. 

1. A method for stacking flat mail items, whereby the mail items are transported with a transport means in turn into stackers and are stacked in a vertical position in these, in each stacker at a stacking point a mail item to be stacked is transported by means of a stacking roll which can be displaced from a stack against a spring pressure, which is controlled to be driven in a start-braking mode, against a stop wall, and a controlled driven underfloor belt and a stacking support coupled so that it can be released from this or driven separately is moved away from the stacking point as the stack grows, the mail item to be added to the stack in each case arrives at an acute angle to the previous, already stacked mail item, before the stacking the mass of each mail item to be stacked is determined, stacking pressures are measured with a measuring device arranged in an area of the stacking roll in a mail item transport plane or on the stacking roll itself, if the stacking roll has engaged with the mail item to be stacked, in a preceding test phase based on statistical investigations the target values for the stacking pressures and braking curves of the stacking roll until the mail items have reached the stop wall are defined as a function of the mass of the respective mail items to be stacked and stored, the respective mail item is braked in accordance with the braking curve of the stacking roll assigned to the mail item mass.
 2. The method of claim 1, whereby a thickness of the mail item to be stacked is determined and for the stacking of a mail item the underfloor belt and/or the stacking support is moved away from the stacking point by the measured thickness of this mail item plus or minus a correction value for setting the target stacking pressure assigned to the determined mass of the mail item as a function of mass and the thickness of the mail item to be stacked and of a spectrum and characteristics of the previously stacked mail item and then, for an actual stacking pressure measured at the stacking roll deviating from the target stacking pressure for this mail item, is moved to maintain the target stacking pressure.
 3. The method of claim 1, whereby for determination of the mass of the mail items, their dimensions are measured, a volume is calculated from this and multiplied by a defined mass factor per volume.
 4. The method of claim 1, whereby the braking curve of the stacking roll runs so that a circumferential speed of the stacking roll at the point when the mail item has reached the stop wall is inverse to the mass of the mail item and whereby the stacking roll is subsequently stopped if the following mail item is far enough away for there to be sufficient time to accelerate the stacking roll to its rated speed, before the mail item has reached it.
 5. The method of claim 1, whereby a circumferential speed of the stacking roll after the stacking of the respective mail item is accelerated immediately to a rated speed of the means of transport if the subsequent mail item is further than a defined distance from the stacker.
 6. A device for stacking flat mail items with a means of transport to transport the mail items in turn into stackers, in which they are stacked vertically, a stacking roll located in each stacker at a stacking point, displaceable away from a stack against a spring pressure, which are driven controlled in starting-braking mode and which transport the mail items up to a stop wall, and with a controlled driven underfloor belt and a stacking support coupled to this so that it can be released or driven separately which is moved away from the stacking point as the stack grows, a measuring device determining the mass of each mail item to be stacked, a measuring device arranged in the area of the stacking roll on the mail item transport plane or on the stacking roll itself for measurement of stacking forces, whereby in a previous test phase, on the basis of statistical investigations, the target values for the stacking pressures and the braking curves of the stacking roll until the mail items have reached the stop wall, depending on the mass of the respective postal item to be stacked are defined and stored, and whereby the respective mail item can be braked by the stacking roll in accordance with the braking curve assigned to the mass of the mail item. 